On June 4, 1982, the Padres had a 5-4 lead in the bottom of the 6th when Pittsburgh loaded the bases with no outs. Gary Lucas, the lefty half of SD's closer tandem, entered the game and struck out Mike Easler, then induced a DP grounder from Dale Berra. Lucas pitched the rest of the game with that same 1-run lead, allowing only 1 baserunner -- a single by opposing pitcher Rick Rhoden.
Lucas's Win Probability Added for the game was 0.803, the highest of any Save for which WPA is known, and probably close to the theoretical limit of WPA in a save. (It is possible to pitch up to 8 innings in a Save, and more IP can produce higher WPA, in general. But the bulk of the value in a very-high-WPA Save comes from escaping an inherited bases-loaded mess in a close game; moving that crisis earlier in the game means lower leverage and so less WPA.)
That game was witnessed by a paid crowd of 5,584 in Three Rivers Stadium. The '82 Pirates had 12 home crowds of less than 5,000, but still drew just over 1 million.
Ironically, Lucas's previous appearance resulted in one of the worst blown saves of the '82 season. After he squandered a 3-run lead in the 9th, his mates scored twice in the top of the 10th -- but Lucas (now in his 4th inning of work) gave up 3 more runs, giving him the loss and a WPA of -0.860.
Oh, and by the way -- the worst WPA ever recorded in a Blown Save was -1.234, by Rawly Eastwick in 1976: